Concrete is a mixture of Portland cement, water and inert materials put in place in a plastic condition but hardening soon after due to the process known as the hydration of the cement. Although concrete is placed in a plastic condition and cannot be tested for quality at the time of fabrication it is now practicable to produce concrete of any quality that may.be necessary to meet the requirements of the work by proper control of the proportioning, making and placing together with subsequent curing.

The fundamental requirements of hardened concrete are strength, durability and economy. Fresh concrete must be workable, that is, it must be of such a consistency and physical make-up that it can be readily placed in the form without segregation of the materials and without requiring an excessive amount of spading to completely fill the form. Uniformity in both the fresh and hardened concrete is necessary to secure economy of materials, to facilitate handling and placing and to obtain uniformity in the completed structure. It is particularly important where water tightness is required.

1 From Design and Control of Concrete Mixtures (Portland Cement Assoc, 1929).

Most concrete is designed on the basis of compressive strength. However, the flexural and tensile strength, the bond with steel reinforcement, and the resistance to wear are in general governed by the same factors which govern the compressive strength. The compressive strength, therefore, may be used as an indication of these other qualities and, since the test for compressive strength is comparatively simple, it is the test that is most often adopted.

If structures are to give long service, the durability of the materials is just as important as the strength. In much of the past practice this has not been fully appreciated and too much emphasis has been placed on strength and economy alone. The most important requirement for durability of exposed concrete is watertightness. Durable concrete requires sound, durable aggregates thoroughly incorporated in a cement paste that is watertight.

While economy of materials is important, there are other factors affecting the economy of concrete which seldom receives proper consideration. The amount of labor required in placing and finishing concrete is a considerable item in the total cost. It can be reduced to a minimum by proportioning the materials to produce a plastic mixture that can be placed easily under the particular conditions of the job. The most economical concrete does not always result from the mix having the lowest cement factor nor the mix with the lowest cost for materials, but rather from the mix for which total cost - materials, handling and placing - is the lowest.

The workability required will be different for different classes of work and will be determined by the methods of transporting and the details of placing - width and depth of forms and spacing of reinforcement. Plastic concrete may be regarded as a mass of aggregate particles, individually floated in a cement paste. This gives a mass that can be transported without segregation and can be placed easily in such a manner that when the forms are removed the hardened concrete will have smooth surfaces, free from honeycombing. Concrete of such consistency will require a minimum amount of finishing. In much of the practice in the past, such concrete has not been obtained. This was largely due to the fact that arbitrary mixtures were specified which did not permit adjustments in the mix to suit the character of materials, the condition in which they were measured or the placing requirements of the job. In the endeavor to obtain workable mixtures, excess water was often added which almost invariably resulted in segregation, porosity and low strength.

Uniformity is important since all parts of a structure designed for the same strength should be made of concrete of the same quality. Moreover, the best economy can be obtained only by the use of uniform batches of concrete. Uniformity is best secured by using plastic concrete made homogeneous by thorough mixing of uniformly measured quantities of materials, including the water.

The above fundamental requirements can be obtained by attention to each of the four major factors which determine concrete quality. These are the use of suitable materials, correct proportioning, careful methods of production, and protection during the curing period.....1

[Note. - The Building Code Committee of the U.S. Department of Commerce has recommended standard specifications governing the use of concrete units (see Minimum Requirements for Small Dwelling Construction; also Properties and Manufacture of Concrete Building Units [U.S. Bureau of Standards], for materials and proportioning).]